- © 2011 by the Seismological Society of America
On 18 April 2008, a moderate-sized earthquake (Mw 5.2) occurred near the Indiana-Illinois state border within 3 km of the Mt. Carmel–New Harmony fault at a depth of 16 km near the northern termination of the Wabash Valley fault system (WVFS). A total of 257 aftershocks were recorded over the next month by a fourteen-station temporary network deployed by Indiana University and University of Memphis/Center for Earthquake Research and Information (CERI). The number of recorded aftershocks is greater than aftershocks recorded from previous earthquakes in the WVFS of similar magnitude within the last 50 years. The number and density of local stations allowed the generation of precise hypocentral relocations using the combination of waveform cross-correlation and joint hypocentral techniques. The relocated hypocenters indicate a well-defined near-vertical fault plane striking 295° and dipping 85° to the north. Analysis of the temporal distribution of aftershocks reveals an anomalously productive aftershock sequence relative to other earthquakes in the region, including four aftershocks with M > 4.0, the largest of which reached 0.6 units of the mainshock, an unusually low b value of 0.579, and Omori law decay parameters of a = –0.662 and p = 1.152. The inferred fault orientation is consistent with the focal mechanism of the mainshock and nearly orthogonal with respect to the trace of the neighboring Mt. Carmel–New Harmony fault. The interpreted fault orientation suggests that the aftershock sequence occurred on a transfer structure near the fault termination. The structure may be related to the change in deformation styles suggested by the transition from the northeast-trending WVFS to the northwest-trending La Salle anticlinorium.